It is the biggest of problems, it is the smallest of problems. At present physicists have two separate rulebooks explaining how nature works. There is general relativity, which beautifully accounts for gravity and all of the things it dominates: orbiting planets, colliding galaxies, the dynamics of the expanding universe as a whole. That’s big. Then there is quantum mechanics, which handles the other three forces – electromagnetism and the two nuclear forces. Quantum theory is extremely adept at describing what happens when a uranium atom decays, or when individual particles of light hit a solar cell. That’s small.

Now for the problem: relativity and quantum mechanics are fundamentally different theories that have different formulations. It is not just a matter of scientific terminology; it is a clash of genuinely incompatible descriptions of reality. more...